1. Field of the Invention
The present invention relates to a surface acoustic wave device having a piezoelectric thin film, a frequency filter, a oscillator, an electronic circuit and an electronic apparatus, and more specifically, to a surface acoustic wave device having a silicon substrate and a potassium niobate piezoelectric thin film, a frequency filter, a oscillator, an electronic circuit, and an electronic apparatus, which are employed in the telecommunications field.
2. Description of the Related Art
Surface acoustic wave devices employing a piezoelectric material with a high electromechanical coupling coefficient (denoted as xe2x80x9ck2xe2x80x9d hereinafter) have been desired in order to improve the performance of non-lead containing surface acoustic wave devices. Lithium niobate is known conventionally as a material with a high k2, demonstrating a k2 of 5.5% using Rayleigh waves. However, it has been shown that k2 can exceed 50% with potassium niobate (xe2x80x9cKNbO3xe2x80x9d hereinafter), as disclosed in Electronics Letters, Vol. 33, No. 3 (1997), p. 193, and much attention has been focused on this area in recent years. In addition, research has been conducted into surface acoustic wave devices employing a KNbO3 thin film, such as disclosed in Japanese Unexamined Patent Application, First Publication No. 10-65488.
However, conventional surface acoustic wave devices have the following problems.
Namely, it is difficult to produce high quality, large KNbO3 single crystals in a surface acoustic wave device that uses a KNbO3 single crystal. As a result, this is not practical from the perspective of mass production. On the other hand, in a surface acoustic wave device employing a KNbO3 thin film, acoustic velocity, k2 and other such characteristics depend on the KNbO3 crystal orientation. Thus, the orientation of the KNbO3 thin film must be controlled. As disclosed in Japanese Unexamined Patent Application, First Publication No. 2000-278084, it is known that a KNbO3 (010) epitaxial film can be obtained by using a (110) oriented substrate of strontium titanate (SrTiO3 hereinafter). This indexing assumes that the b axis lattice constant is the largest. However, even if the orientation of the KNbO3 thin film can be controlled using a SrTiO3 substrate, it is difficult to form a SrTiO3 substrate which is larger than two inches. Accordingly, this also is not suitable from the perspective of mass production. Furthermore, even if such a substrate could be produced hypothetically, it is not viewed to be practical in terms of cost.
A silicon (denoted as xe2x80x9cSixe2x80x9d hereinafter) substrate would appear promising from the perspective of cost and capacity for mass production. Still, it is difficult to obtain a high-quality epitaxial thin film even when the KNbO3 thin film is formed directly on top of the Si substrate, because of lattice mismatches and the like. As a result, a high k2 cannot be obtained.
It is the objective of the present invention to resolve the above-described problems by providing a surface acoustic wave device having a high k2, which element is manufactured by employing a Si substrate that is advantageous in terms of cost and capacity for mass production, wherein a high quality KNbO3 epitaxial thin film is formed onto the Si substrate.
The first aspect of the present invention is a surface acoustic wave device having a (110) silicon substrate, and a (010) potassium niobate piezoelectric thin film. This surface acoustic wave device has a first oxide thin film layer formed on top of the silicon substrate, a second oxide thin film layer formed on the first oxide thin film layer, a potassium niobate piezoelectric thin film formed on top of the second oxide thin film layer, and a protective thin film comprising an oxide or nitride formed onto the potassium niobate piezoelectric thin film.
As a result of the above design, the first oxide thin film layer and the second oxide thin film layer can be made to undergo epitaxial growth in sequence on top of the silicon substrate. A high quality KNbO3 epitaxial thin film can then be formed on top of the aforementioned layers. Specifically, it becomes an easy matter to form a (010) KNbO3 epitaxial thin film, making it possible to provide a surface acoustic wave device having a high k2 which is advantageous with respect to cost and capacity for mass production.
The first oxide thin film layer is preferably formed from strontium oxide (denoted as xe2x80x9cSrOxe2x80x9d hereinafter) or magnesium oxide (denoted as xe2x80x9cMgOxe2x80x9d hereinafter). These first oxide thin films are capable of epitaxial growth on top of the (110) silicon substrate, ultimately enabling epitaxial growth of KNbO3. Specifically, a (010) oriented KNbO3 is easily formed.
The second oxide thin film is preferably formed of SrTiO3.
SrTiO3 is capable of epitaxial growth on the aforementioned first oxide thin film, and enables epitaxial growth of KNbO3 on the SrTiO3 film. Specifically, a (010) oriented KNbO3 is easily formed.
The second aspect of the present invention is a surface acoustic wave device having a (100) silicon substrate and a (010) KNbO3 piezoelectric thin film. This surface acoustic wave device has a first oxide thin film layer formed on top of the silicon substrate, a second oxide thin film layer formed on top of the first oxide thin film layer, a KNbO3 piezoelectric thin film formed on top of the second oxide thin film layer, and a protective thin film comprising an oxide or nitride formed on top of the KNbO3 piezoelectric thin film.
As a result of the above design, the first oxide thin film layer and the second oxide thin film layer can be made to undergo epitaxial growth in sequence on top of the silicon substrate, and a high quality KNbO3 epitaxial thin film can then be formed on top of these layers. In particular, formation of a (010) KNbO3 epitaxial thin film becomes an easy matter, making it possible to provide a surface acoustic wave device having a high k2 which is advantageous with respect to cost and capacity for mass production.
The first oxide thin film layer is preferably formed from cerium oxide (denoted by xe2x80x9cCeO2xe2x80x9d hereinafter), zirconium oxide (denoted as xe2x80x9cZrO2xe2x80x9d hereinafter), or yttrium-stabilized zirconia (denoted as xe2x80x9cYSZxe2x80x9d hereinafter).
These first oxide thin films are capable of epitaxial growth on a (100) silicon substrate, so that epitaxial growth of KNbO3, and (010) KNbO3 in particular, is made possible in the end.
The second oxide thin film layer is preferably formed from strontium titanate (denoted as xe2x80x9cSrTiO3xe2x80x9d hereinafter).
SrTiO3 is capable of epitaxial growth on the first oxide, and enables epitaxial growth of KNbO3, and (010) KNbO3 in particular, on the SrTiO3 film.
The second aspect is preferred over the first aspect in that, during formation of the oxide layer, a relatively high-temperature, high-vacuum is not required, and because a (100) silicon substrate is more readily available and less expensive than a (110) silicon substrate.
In the present invention""s surface acoustic wave device, an electrode is formed on top of the piezoelectric thin film or the protective thin film. However, when providing an electrode on top of the piezoelectric thin film, there is some concern that the piezoelectric thin film may be degraded by water or the like during the electrode forming process. Further, when forming the protective thin film on top of the piezoelectric thin film and the electrode, it is necessary to take out the electrode through the protective thin film. As a result, formation of the frequency filter becomes troublesome and costs rise. Accordingly, it is preferable that the electrode be formed on top of the protective thin film.
This is also true of the frequency filter and the oscillator that will be described below.
The third aspect of the present invention is a frequency filter characterized in the provision to any one of the above described surface acoustic wave devices of a first electrode, which is formed on top of the protective thin film or the piezoelectric thin film; and a second electrode, which is formed on top of the protective thin film or the piezoelectric thin film; the second electrode resonating at a specific frequency or at a specific band frequency of the surface acoustic waves that are produced in the piezoelectric thin film from an electric signal impressed by the first electrode, and converting this resonance to an electric signal.
Due to the high k2 provided by this design, it is possible to provide a frequency filter having a broad relative bandwidth.
The fourth aspect of the present invention is a oscillator characterized in the provision to any one of the above described surface acoustic wave devices of an electric signal impressing electrode, which is formed on top of the protective thin film or the piezoelectric thin film and which generates a surface acoustic wave on the piezoelectric thin film from the impressed electric signal; and a resonating electrode, which is formed on top of the protective thin film or the piezoelectric thin film and which resonates a specific frequency or specific band frequency of the surface acoustic waves that are generated by the electric signal impressing electrode.
Due to the high k2 of the surface acoustic wave device""s piezoelectric thin film in this design, it is possible to eliminate the expander coil. Thus, a oscillator having a simple Circuit structure can be provided. Moreover, integration with an IC becomes possible, so that a small-sized, high functioning oscillator can be provided.
The fifth aspect of the present invention is an electronic circuit characterized in the provision of the above described oscillator and an electric signal supplying element for impressing an electric signal on the electric signal impressing electrode provided in the oscillator; wherein the electronic circuit selects specific frequencies from the frequencies of the electric signal or converts the electric signal to specific frequencies, or applies specific modulation to the electric signal and carries out specific demodulation or specific wave detection.
As a result of this design, the k2 of the piezoelectric thin film forming the surface acoustic wave device that is proved in the electronic circuit""s oscillator is high, and integration with an IC is possible. Thus, a small-sized, high-functioning electronic circuit can be provided. The sixth aspect of the present invention is an electronic device characterized in including at least one of the above-described frequency filter, oscillator, and electronic circuit. The k2 of the piezoelectric thin film in the electronic device is high in this design. As a result, it is possible to offer a small-sized, high-functioning electronic device.